I posted a "fix" a while back for this problem and it seemed to work.

You can find it here: http://androidforums.com/zte-avid-4g/691152-screen-rotation-problem.html

But as my phone got older the problem came back and was worse. It seems to be an accelerometer defect, which is a manufacturer defect and is covered by a 12 month manufacturer warranty.

So I would suggest just bringing your phone back to the store you bought it from. They will replace it for free.